As an optical information recording-reproduction apparatus records or reproduces information to or from a track on an optical disk, the apparatus performs tracking operations wherein a spot of light, produced by converging a laser beam emitted from a semiconductor laser by means of an objective lens, is irradiated to the track.
Conventionally, a light beam guiding track of the optical disk is composed of "groove" sections and sections between grooves (hereinafter, referred to as "land" sections).
In a conventional optical information recording-reproduction apparatus arrangement, where if an attempt is made to record information on both land sections and groove sections of an optical disk, whether information recorded on the land sections is reproduced, leakage of the information recorded on the adjacent groove sections (i.e., "crosstalk") increases. Likewise, when information recorded on the groove sections is reproduced, crosstalk from information recorded on the adjacent land sections increases. This adversely effects the quality of the reproduced signal. For this reason, information was conventionally only recorded on or only reproduced from either the land sections or the groove sections of optical disk media. Therefore, in the above-mentioned conventional optical information recording-reproduction apparatus, it was necessary to perform focusing and tracking on only one section of a track, i.e., either the land sections or the groove sections.
However, this arrangement imposed a serious limitation on the amount and density of information which could be stored on optical disk media, present, various alternative methods have been proposed to comply with demands for an optical disk having higher recording density to capabilities. One such method is disclosed in Japanese Unexamined Patent Publication No. 5-81717/1993 (Tokukaihei 5-81717), in which a magneto-optical disk (i.e., one utilizing magneto-optical effects) is utilized for storing and reproducing information at greater densities than that capable of being read using other conventional optical techniques. The proposed method makes use of what is known as magnetic super resolution properties. Using these properties crosstalk between adjacent tracks can be decreased by using "masking information", which is recorded on a magnetic layer formed on a perpendicular magnetization film, and thereby allowing a higher density of stored information.
In another approach, an apparatus which records or reproduces information on or from both the land sections and groove sections by switching the polarity of a tracking servo circuit is disclosed in Japanese Examined Patent Publication No. 4-27610/1992 (Tokukohei 4-27610).
The methods and apparatus disclosed in the above Publications decrease crosstalk between adjoining tracks even when information is recorded on or reproduced from both the land sections and the groove sections. As a result, since information can be recorded on or reproduced from both the land sections and the groove sections, at least twice the conventional amount of information can be recorded and recording with high information density becomes possible.
In a conventional optical information recording-reproduction apparatus, since focusing is performed on only either the land sections or the groove sections, the apparatus can perform suitable recording or reproduction using an independent focus servo control signal. However, in an optical information recording-reproduction apparatus for recording and reproducing information on or from both the groove sections and the land sections that uses the same focus servo control signal for recording/reproducing signals for both the land sections and the groove sections, it is not possible to achieve a satisfactory recording or reproduction. With reference to FIGS. 6(a) and 6(b), the following reasons describe why the most suitable focus servo control signal is different for the tracking of land sections and the tracking of groove sections.
On an optical disk 6, groove sections 6a and land sections 6b, which are depicted as convex sections between groove sections 6a, are formed on a disk substrate. The waveforms shown in FIG. 6(a) represent a focus error signal, F, and a tracking error signal, T, which are servo error signals obtained from an optical pickup when only a single focus servo is actuated. With respect to focus error signal F, upward peaks of the depicted waveform are formed when a spot of a semiconductor laser beam converged on optical disk 6 by an objective lens comes to land sections 6b. Likewise, when the spot of the semiconductor laser beam comes to groove sections 6a, the bottom peaks of the depicted waveform are formed. Focus error signal F is usually affected by a track of the optical disk 6 and it has a period same as that of the tracking error signal T shown in FIG. 6(a). Moreover, the focus error signal F has 90 degrees phase difference from the tracking error signal T and is modulated. Such a change in the focus error signal F is referred to as crosstalk between error signals in the present invention.
A reason that the crosstalk between error signals shown by waveforms in FIG. 6(a) is disclosed in Japanese Examined Patent Publication No. 5-68774/1993 (Tokukohei 5-68774). The crosstalk between error signals occurs because in the photo detector apparatus, which generates a servo error signal, light reflected from the optical disk is affected with aberrations due to the optical components of the optical pickup and, in particular, to the objective lens. Consequently, an asymmetrical property is attributed to the reflected light and, thus, the tracking error signal leaks into the focus error signal. Due to this crosstalk, the focus error signal F acquires a periodic form same as tracking error signal T, but its phase is delayed by 90 degrees due to the asymmetric properties of the light reflected from the optical disk.
As is clear from FIGS. 6(a) and 6(b) in the case where the tracking servo is turned ON after the focus servo, when the land section 6b is tracked by the spot of the semiconductor laser beam, its focusing point is on line L and when the groove section 6a is tracked, its focusing point is on line G due to the crosstalk between error signals F and T.
Since the focusing point at the time of tracking the land section 6b is different from that at the time of tracking the groove section 6a, whenever focus servo control is implemented using a the focus servo control signal generated according to the sole focus error signal F for both land section 6b and groove section 6a, deviation in the optical axis direction between. the objective lens and the optical disk 6 (i.e., focus offset) occurs. As a result, shapes of the converged light spot on the optical disk 6 become different, such that information respectively recorded on land section 6b and groove section 6a cannot be satisfactorily recorded or reproduced. Referring to FIGS. 6(a) and 6(b), it is evident that if the deviation between GND and line L for the land section focus offset is amount "1" and the deviation between GND and the line G for the groove section focus offset is amount "g", then the total deviation amount of the focusing point between the tracking of the land section 6b and the tracking of the groove section 6a is represented by 1+g.
A method for controlling crosstalk between the error signals by adjusting rotation of the objective lens about its optical axis is disclosed in Japanese Examined Patent Publication No. 5-68774/1993 (Tokukohei 5-68774). However, using that method requires a lot of time to adjust the objective lens. Moreover, since there exists not only the aberration of the objective lens but also aberration of other optical parts, the crosstalk between error signals cannot be completely eliminated by that method. Consequently, deviation of the focusing point along the optical axis between tracking land sections and tracking groove sections cannot be completely eliminated, thereby making it impossible to achieve optimum recording and reproducing when using that prior art method in controlling crosstalk.